Maintenance Q&A
When to spend more and when not to

By Paul Abelson, Senior technical consultant

With all the talk about corrosion and how it eats away at components, should I switch to stainless steel nuts and bolts? I have a 2013 International ProStar pulling a 2015 Great Dane. Both are in pretty good shape and I want to keep them that way.

Even if you can afford the price premium for stainless steel fasteners, the answer is probably not. It is widely believed that stainless steel is stronger than the mild carbon steels used in most fasteners, but it really isn’t. Stainless steel costs at least twice the price of ordinary fasteners, and its strength is somewhere between grade 2 and grade 5 fasteners, leaning closer to grade 5.

The Society of Automotive Engineers issued a recommended practice, J429, dealing with properties of steel fasteners. It covers nuts and bolts, studs and U-bolts up to 1 1/2 inches in diameter.

Bolts are graded by yield strength and tensile strength among other properties. Fasteners work by stretching within their elastic limits and clamping the items being held together between the head and nut. As long as the limits, which are measured in pounds per square inch (psi), are not reached, the fastener will hold. If they are exceeded, the fastener will stretch and permanently lose its clamping ability.

That’s why it is important to apply the torque specified in repair manuals and to avoid using “just a bit more to be sure.” Over-torquing is the major cause of fastener failures in trucks, according to the Technology and Maintenance Council.

SAE classifies automotive fasteners into grades. The most common are 2, 5 and 8. They are identified by lines on the bolt’s hex-head: no lines for grade 2, three lines pointing to alternate points on the head for grade 5 and six lines, one at each corner, for grade 8. The bolts and nuts are made of low carbon steel, medium carbon steel, and medium carbon alloy steel, respectively. There are stronger fasteners beyond grade 8, used in specialized applications only.

Most grade 8 fasteners are cadmium plated for corrosion resistance and ease of work. Some grade 2 and 5 fasteners are dipped in zinc or plated for corrosion resistance. Grade 2 fasteners have a minimum yield strength as low as 36,000 psi. For the stronger fasteners, grades 5 and 8, those values are 81,000 psi and 130,000 psi, respectively.

Depending on the chemistry and dimensions of the SS fasteners, yield strength can be between 20,000 and 55,000 psi. If there is any structural load involved, use the appropriate grade 5 or 8 fastener recommended by the component or equipment manufacturer and torque it to specification.

If you are attaching a stainless tool box or other lightly stressed or decorative accessory, by all means consider SS, but they’re not for heavy-duty fastening. To ensure maximum corrosion resistance, nothing beats frequent inspections and a good coat of paint. In areas that collect road spray, consider using rust-proofing paint. You may have to clean an area with a wire brush and prime it before painting.

For more on fasteners, look up Fastenal’s Technical Reference Guide s7028, American Fastener Technologies Corp., Lawson Products, Imperial Supplies and others.

I have a 2013 Pete with a 450 hp motor and 10-speed transmission. It has about 280,000 miles hauling mostly dirt and rock from construction sites. I’ve developed a vibration that seems to be in the drive wheels. I took it to a friend’s shop. He checked the wheels and tires for balance and runout and the drive shafts for balance. He inspected all the U-joints and they seemed fine. He checked other stuff and couldn’t find anything wrong. Do you have any ideas?

There are several possibilities. When we spoke, you mentioned that due to the heavy-duty, short-haul work you do, you recently had your clutch replaced. Rather than using an OEM or equivalent replacement, your friend “saved you money” by installing a one-size-fits-all clutch. While many of these replacements are dimensionally correct, they often aren’t engineered to absorb the torsional vibration generated by modern engines, especially at low rpm.

Torsional vibration is created when combustion takes place and the piston pushes down to turn the crankshaft. In a six-cylinder engine, that creates a series of impacts, one every 120 degrees of crankshaft rotation. Each time the crankshaft rotates, it gets three impacts that twist it forward and three periods when it springs back between impacts. The strength and elasticity of the crankshaft absorb these impulses, but transmit these regular impulses as vibration. Since it comes from rotary motion, it is called torsional vibration.

A clutch designed for these forces uses springs to absorb and cushion the torsional vibration. If not properly absorbed, the vibrations carry through the entire drive train. These repeated impacts over time can damage transmission and differential gears, synchronizer parts, U-joints, bearings and even suspension parts.

Small shops and dealers stock these will-fit parts to keep their inventory costs down, but there’s nothing like a properly engineered part.

Another possibility is a failing vibration damper. They are mounted on the crankshaft. They use counterweights in a silicone-based viscous fluid to absorb the impulses. Silicone is subject to hardening due to impact or just plain age. Your occupation leaves you vulnerable to early damper failure.

It is also possible that when your clutch was replaced, fasteners were torqued with an air wrench and torque-limiting device rather than hand-tightened with a torque wrench. These “torque sticks” reduce torque transmission but do not limit it. Fasteners torqued beyond their yield point allow fastened components to loosen, causing vibration. LL

(Editor's note: When sending in questions for Maintenance Q&A, please include the make and model of your truck and your vehicle identification number (VIN) as well as your contact information. Paul Abelson tries to respond to every question, whether it's published or not. Send questions to TruckWriter@WowAccess.net)